how to satisfy requirements of an ancient X-client (4-plane overlay visual)

[Robert Urban]

Hello,

I'm struggling to satisfy the X11 requirements of an ancient application on
modern graphics hardware.

The application runs on Digital UNIX V4 and is about 20 years old. It expects
generally 8-plane pseudocolor visuals, but with one decisive exception:
It requires a 4-plane overlay visual, and will take nothing else.

My impression is that modern GPUs long since stopped offering 4-plane visuals.
Also it seems that the X-server can only offer an overlay visual if the GPU:

1) offers a corresponding visual, and
2) supports overlays.

Is this correct, or can the X-server somehow work around the lack of one or the
other capability?

Can anyone suggest a way of satisfying these ancient requirements? I don't mind
acquiring hardware, if that will help.

thanks,
Rob